In fact, just six counties in the two states accounted for 51 percent of the more than 16 million trees harvested nationwide in 2012, the most recent year government data is available. In all, North Carolina and Oregon trees constituted 79 percent of that year’s harvest. Industry insiders say the trend hasn't changed.

The country’s biggest Christmas tree provider is Ashe County, a mountainous region in North Carolina bordering Tennessee and Virginia. Tree farmers in Ashe, population 28,000, harvested nearly 2 million trees in 2012.

"It's a critical piece of our local economy," said Travis Birdsell, a tree farmer in Ashe who estimated that 250 farms are active in the county. “It’s certainly the largest sector.”

While Christmas trees farms operate across the country, most pale in size to those in Oregon and North Carolina, according to Doug Hundley, a spokesman for the National Christmas Tree Association. Hundley said that growers in most states operate choose-and-cut farms, small operations where consumers pick their own trees to cut.

"They do a little shipping overstate, but nothing like what Oregon and North Carolina do," Hundley said.

Tom Norby operates the Trout Creek Tree Farm, an 80-acre grove in unincorporated Multnomah County, Oregon, just north of Clackamas County and a half-hour’s drive east of Portland. Norby attributes the size of the Christmas tree industry in the two states to climate and geography.

"Where can you grow trees?" Norby said. "Oregon has a climate that supports the types of trees you'd want for Christmas trees up there in the hills. North Carolina does, too. It's all of those mountain states."

Advocates for live Christmas trees are quick to talk about their state’s signature tree. In Oregon, it’s the Noble fir, which has a blue tint and grows at high altitudes. In North Carolina, it’s the Fraser fir, an evergreen that was the most-harvested tree nationwide in 2014.

The most common misconception about Christmas trees is that they come from forests, farmers say. Instead, trees are cultivated for the specific purpose of being harvested for holiday decor. Farms come in all sizes, from family-run spare-land operations to companies with thousands of acres devoted to the stock.

Trees take eight to 10 years to grow to salable size. During those years the trees must be fertilized, sheared and tended to for insects and weeds.

"It's a long-term product," said Birdsell, adding that farmers have a short window. Christmas trees are typically harvested in November, with Thanksgiving marking the start of the busy season.

The real Christmas tree industry has faced a tough decade, farmers told NBC News, as an oversupply of trees planted in the late 1990s and early 2000s, combined with a slow economy, pressured farmers to keep costs down. The downturn in sales led them to plant fewer seedlings, which has resulted in a small crop size for the 2018 harvest.

While tree prices vary from $25 to $300 based on size, species and quality, last year the average tree sold for $75, according to an NCTA survey.

But the industry's biggest threat is from artificial trees. Hundley said that 75 percent of trees put up in homes in the 2017 holiday season were artificial. This affects farmers in two ways: A buyer of an artificial tree is likely to hold onto it for years, thus removing a potential real tree customer from the market. Fewer tree buyers also leads to farmers planting fewer seedlings, which leads to constrained stock later on.

"We are selling less real trees than we were 40-50 years ago despite a major increase in population," Hundley said. "The artificial tree has bitten severely into this industry.”

Graphics sources: USDA / National Christmas Tree Association

CORRECTION (Dec. 12, 2018, 4:50 p.m. ET): An earlier version of this article misstated what type of tree the Fraser fir is. It's a fir tree, not a pine.